The present invention relates generally to automotive electronics systems and more particularly to low quiescent current circuitry for awakening such systems from an inactive or sleep state and controlling the distribution of power within such systems.
The electrical system in today's automobile is called on to monitor and control a plethora of vehicle convenience, safety and operating functions. Typically these functions are partitioned and performed by numerous multiplexed control modules distributed throughout the vehicle. While these modules may be physically remote from each other, they eventually all derive their electrical power from a common limited source: the vehicle battery. Because an unlimited power source is not available to perform these various functions, power management to prevent premature discharge of the vehicle battery is a key design consideration.
Many schemes have been developed in the past to minimize current drain and conserve battery power by providing a sleep or inactive state for the system. In such systems, however, an issue then arises of how and when to awaken the system from its sleep state. Again, many schemes have been devised. Typical synchronous approaches utilize timing circuits or counters in combination with polling routines to periodically interrogate switch status. However, some asynchronous approaches have been known.
For example, U.S. Pat. No. 4,839,530 provides for activation of a master unit upon closure of an activation switch connected to a slave module by temporarily applying an activation voltage level different from the active mode operation voltage on a signalling link. To generate this activation voltage, each slave unit capable of activating the system has activation drive means that connect to corresponding power supply control means in the central master unit.
The inventor herein has recognized a number of problems associated with this prior art approach. For example, should an activation switch become stuck in a closed position, the system would continually remain in the active mode drawing current from the battery at its normal operating level thereby defeating the purpose of the voltage activation circuitry. Also, the system disclosed may be activated by noise on the switch contacts because it may not distinguish between an actual switch activation and the presence of electrical noise on the switch contacts.